1. Field of the Invention
The present invention relates to a semiconductor module and a semiconductor device using a power semiconductor element, and more particularly relates to a semiconductor device which is useful, for example, for inverters and converters.
2. Description of the Related Art
In various kinds of motor control, GTOs (Gate Turn Off Thyristors) have been used for large power applications and transistors have been used for small power application. However, in recent years IGBTs (Insulated Gate Bipolar Transistors) are rapidly spreading in the field in place of GTOs and conventional transistors because of ease of use brought about by the ability of these devices to control a large current with a voltage signal. IGBTs are generally used in the form of modules. Today, there are various types. For instance, in three-phase motor control, three phase and upper-arm/lower-arm currents need to be switched. That is, three switches are used for the upper arms, and three switches are used for the lower arms, such that a total of six switches are required. Therefore, there exists a type of device where IGBTs corresponding to one arm are mounted on a module, a type of device where IGBTs corresponding to six arms, are mounted on a module, and a type of device where an additional circuit is further mounted.
As for structures, although various have heretofore been proposed ideas, an example close to the present invention, in which IGBTs corresponding to one arm are mounted on a module, will be explained below.
Since plural IGBT chips are connected in parallel when a single chip cannot control a desired amount of current, a semiconductor switching device mounting plural chips connected in parallel will be discussed.
A structure will be described below, according to a common manufacturing process. One surface of an IGBT chip is bonded on one of a plurality of Cu thin plates, which are bonded on both surfaces of an alumina or A1N ceramic substrate, using a solder having the highest melting point among solders to be used in the module. This Cu plate commonly becomes a collector terminal. On the other surface of the chip, an emitter and a gate electrodes are formed. Both are wire-bonded to an emitter and a gate terminal of Cu thin plates formed on the same surface of the ceramic substrate as the collector terminal is formed, respectively. The Cu plate on the other surface of the ceramic substrate is bonded to a cooling plate as a base of the module using solder. An Al or Cu plate is commonly used for the cooling plate. Connection of the module external terminals to the electrodes on the ceramic substrate is performed by Cu leads integrally formed together with the external terminals. The external terminal is usually referred to as a terminal block. A molded resin case is bonded to the metallic base (cooling plate) using an adhesive. A gel is injected through an aperture intentionally opened between the case and the terminal block and hardened, and then above it a hard resin is injected and hardened. It is basically preferable to harden the gel after sufficiently removing bubbles from the injected gel. However, if bubble-removing operation is performed in this structure, the gel can rise up along the inner surface of the case to cause degradation of bonding between the hard resin and the case. Therefore, the hard resin is often injected without the important removal of bubbles.
The above is a common manufacturing process and a common structure of a module.
The module is attached to a proper cooling structure with bolts using holes formed on the four corners of the metallic base. Since the electric potential of the cooling structure is generally at ground potential, insulation to the IGBT chip is performed by the ceramic substrate.
The external terminals are composed of a collector terminal, an emitter terminal, a gate terminal and an emitter auxiliary terminal for the gate.
In the aforementioned conventional technology, there are problems as follows.    (1) The life-time of bonding solder between the metallic base and the ceramic substrate: When a module starts to operate, heat is generated and shear stress is generated in the bonding solder due to a difference of linear expansion coefficients between the metallic base and the ceramic substrate. The solder is thermal-fatigued and then cracks progress inside the solder generally from the periphery to the metallic base. When the cracks have progressed to a certain degree and the thermal resistance between the IGBT chip and the metallic base is increased, the solder cracks progress at an accelerated rate due to the application of positive thermal feed-back on the solder cracks and finally the module becomes incapable of operating.    (2) The gel plays a role of passivation by coating over the IGBT chip. When the hard resin allowed to flow over the gel is hardened, the chip is usually heated at nearly 150° C. At this time, the volume of the gel is expanded by approximately 10%. In the process of cooling after completion of hardening, cracks are generated inside the gel because the contraction of the gel volume is restricted by the case and the hard resin. This phenomenon can be confirmed by observing an actual product with X-ray inspection. If the cracks reach over the IGBT chip, the passivation effect for the chip disappears.    (3) In a case of using modules connected in parallel, the gate-emitter circuit in the input circuit forms a loop. There are some cases where an oscillation phenomenon occurs due to inductance and floating capacitance between the gates and between the emitters and input capacitance.    (4) The external terminals are generally arranged in the lateral direction of the module in the order of the collector terminal, the emitter terminal, the gate terminal and the gate/emitter auxiliary terminal for structural reasons. In this arrangement, however, the external wiring becomes complex and an erroneous operation is likely to occur due to mutual noises when a large number of modules are mounted as an inverter.
Especially in a case where the modules are applied to an inverter for a vehicle, the mounting space in a vertical direction should be decreased to be as low as possible since the inverter is installed under the floor. Therefore, the modules are preferably mounted by arranging the shorter side direction of the modules in the vertical direction.
In this case, the wiring becomes complex when the conventional arrangement of the terminals is employed.
    (5) In order to switch the modules at a high speed, inductances of the collector and the emitter should be decreased to be as small as possible.
Although the above description regarding modules refers to problems of a power switching devices, these are basically common problems for current control devices using semiconductors. In the conventional technology, an insulator capacitance component is generated at the portion where withdrawal or a void is generated inside the solder bonding the metallic film and the insulator substrate, or at the portion where a gap is provided between the metallic film and the insulator substrate. This insulator capacitance component is connected in series to an insulator capacitance component due to the insulator substrate. When a high voltage is applied to the module is this case, a partial discharge (corona discharge) occurs in the vacant layer where the insulator capacitance component occurs.
Since the partial discharge during operation of the module deteriorates the filling agent inside the module such as silicon gel, deterioration of insulation is caused as an end result. The partial discharge during switching causes noise, and the noise causes error operation especially in a module of insulated gate type element such as an IGBT.
However, when a number of semiconductor elements are formed in a module, it is difficult to make the amount of current flowing in each of the elements uniform due to variations of characteristics of each element and differences in wiring length inside the module. When the non-uniformity in current occurs among the elements, spike noise occurs due to shift in ON/OFF time among the elements during switching operation. There arises a problem in that solder or the metallic wire of one element where current is concentrated is deteriorated in a short time compared to the other elements.